This invention relates to a spot scanning system capable of mapping an underwater area. In the line scanning prior art methods, every part of the line has the same intensity, for all practical purposes. However, in underwater scanning, this is not true because of the rapid attenuation of a light beam in water. A point in the middle of any line would reflect light of much greater intensity than light reflected from either end of the same line, since the distance traveled by the incident beam of light to the middle of each line is considerably less than to the ends of the same line, and hence the light intensity is attenuated less in the middle of the line. Since light attenuates exponentially underwater, this factor must be taken into consideration in any system used for underwater viewing.
This invention relates to a compensator which is used with an apparatus which optically maps underwater surfaces, may display the mapped image in real time on a cathode ray tube, or store the image in a video tape recorder. In operation, the system may be attached to either a towed or self-propelled underwater vehicle.
The apparatus with its compensator is an improvement over the Apparatus for scanning an underwater area, described in U.S. Pat. No. 3,775,735, to Funk et al, which issued on Nov. 27, 1973. Both inventions have the same assignee. Both inventions also involve a scanning spot which rotates in a circle or an arc of a circle.
The apparatus of the invention just mentioned in the paragraph immediately hereinabove, as well as of this invention, includes the following basic components: (1) a point light source which is spectrally optimized for underwater transmission; (2) collecting and collimating optics for the light source; (3) a dual prism and lenses mounted in a rotating cylinder; (4) receiving optics, for receiving the beam of light reflected from the target; (5) a photomultiplier tube, for converting the received beam of light into an electrical signal; (6) automatic gain control electronics, to compensate for the variable attenuation of the incident and reflected light beams; (7) a display cathode ray tube; or (8) a magnetic tape recorder, to display or record the target area; (9) a power supply, to supply energy where needed; and (10) underwater housings.
The invention specifically described herein relates primarily to item (6).